In subterranean well operations, it is necessary from time to time to set a whipstock in a subsurface well conduit such as a tubing string or a well casing. The whipstock is set to deviate a mill bit away from the longitudinal axis of the conduit to mill a window in the conduit. Thereafter, a drill bit is passed through the window to drill a deviated wellbore at an angle to the longitudinal axis of the conduit.
The cost and time consumed in using a conventional rotary drilling rig in the foregoing situation is considerable and there has been a trend toward the use of coiled tubing units for these and other well operations heretofore conducted with conventional (jointed straight pipe) drilling rigs.
Coiled tubing units are known in the art, but not widely used in the field yet. Coiled tubing units are nevertheless available on a commercial basis. Inventions such as that disclosed herein will render coiled tubing units more readily useful in the field by reducing both the cost and time expenditures, as compared to a conventional drilling rig, for a given operation.
One beneficial use for a coiled tubing unit is the situation where a wellbore is completed with a first casing that lines the wellbore and then concentric within that first casing a second casing or tubing of a smaller diameter is disposed, but the second casing terminates before the wellbore or first casing terminates. In this type of completion there is a substantial portion of the wellbore and first casing exposed below the end of the smaller diameter, second casing. Coiled tubing is extremely useful in such a situation because the coiled tubing and whatever tools it carries can be inserted into the wellbore through the second, smaller diameter casing; exit from the second casing into the wider diameter first casing; and the tools on the coiled tubing operated within the larger diameter first casing. With coiled tubing units this sort of operation can be carried on without undergoing the considerable time and cost required for removing the smaller diameter second casing before being able to operate in the well below the second casing. This invention allows for a procedure wherein the second, smaller diameter casing is left in place in the well, the whipstock and associated equipment is lowered at the end of coiled tubing through the smaller diameter, second casing until it exits the second casing and then the whipstock is set inside the larger diameter first casing.